一、4G网络中TAC 在4G(LTE)网络中每个基站(eNB)下有多个小区(Cells),多个小区组构成一个跟踪区(TAC-Tracking Area Code)。
TAC(Tracking Area Code)标识特定网络内的跟踪区域,TAC与PLMN-ID结合起来构成全球唯一的标识TAI(Tracking Area Identity)。
二、4G中终端TAU 接入4G(LTE)的移动终端(UE)在完成附着(Attach Procedure)进入EMM-REGISTERED状态;当移动到新TAC时触发Tracking Area Update(EMM-EPS移动性管理)。
在RRC Idle状态或RRC Connected状态下终端(UE)都可以触发Tracking Area Update流程;但只有当UE处于RRC Connected状态时TAU流程才会结束。终端TAU流程如下图所示:
图1.4G网络中TAU全部信令流程三、周期性TAU(Periodic TAU)即使处于TAI列表中的TA内,处于空闲状态的终端(UE)也会周期性地向MME发送TAU请求消息(TAU Request message),以确保终端(UE)能够响应网络的寻呼,进行数据传递。
四、终端TAU信令流程处于空闲态下的终端(UE)当需要进行TAU时,首先在Uu口建立RRC连接,经S1接口向核心网发送终端(UE)初始信息,网络侧接受终端的TAU请求后由Uu口通知终端。
图2.4G终端TAU主要消息
4.1. rrcConnectionRequest
message t = 1 u c1 t = 2 u
rrcConnectionRequest(RRC连接建立请求)
criticalExtensions
t = 1 u rrcConnectionRequest_r8(RRC连接建立请求-R8)
ue_Identity t = 2 u(UE ID )randomValue numbits = 40(随机接入值=40位比特)
data = 88 dc fb 6b 88(HEX)
establishmentCause = 3 : EstablishmentCause_Root_mo_Signalling (RRC建立原因:MO-Sig)
spare numbits = 1 data = 0(BIT)
4.2. ueCapabilityEnquiry
message t = 1 u c1 t = 8 u
ueCapabilityEnquiry(UE能力查询)
rrc_TransactionIdentifier = 2
criticalExtensions
t = 1 u c1 t = 1 u
ueCapabilityEnquiry_r8(UE能力查询-R8)
tOptFlags
nonCriticalExtensionPresent = 0
ue_CapabilityRequest(UE能力请求)
n = 4
elem[0] = 0 : RAT_Type_Root_eutra
elem[1] = 2 : RAT_Type_Root_geran_cs
elem[2] = 3 : RAT_Type_Root_geran_ps
elem[3] = 4 : RAT_Type_Root_cdma2000_1XRTT
4.3. DownlinkNASTransport(TAU ACCEPT)
S1apMsg(S1AP消息)
t = 1 u initiatingMessage(初始消息)
ProcCode = 11 : id_downlinkNASTransport(下行NAS传输ID:11)
DownlinkNASTransport
protocolIEs
tOptFlags
handoverRestrictionListPresent = 0
subscriberProfileIDforRFPPresent = 0
mME_UE_S1AP_ID = 83939703
eNB_UE_S1AP_ID = 6926
nAS_PDU
nocts = 49
data = 27 d5 80 d7 ff 02 07 49 01 5a 49 50 0b f6 64 f0 00 01 42 c8 f0 8c cc e8 54 06 40 64 f0 00 35 bf 57 02 20 00 13 64 f0 00 35 bf 23 05 f4 be c3 5e f5(HEX)
4.4. ulInformationTransfer(TAU Complete)
message t = 1 u c1 t = 10 u
ulInformationTransfer(上行消息传输)
criticalExtensions
t = 1 u c1 t = 1 u
ulInformationTransfer_r8
tOptFlags
nonCriticalExtensionPresent = 0
dedicatedInfoType(专用信息类型)
t = 1 u
dedicatedInfoNAS(专用NAS信息)
nocts = 8
data = 27 f0 8b 8b ec 03 07 4a(HEX)
4.5.UEContextRelease
S1apMsg(S1AP消息) t = 2 u
successfulOutcome(成功输出)
ProcCode = 23 : id_UEContextRelease(UE上下文释放ID=23)
UEContextReleaseComplete(UE上下文释放完成)
protocolIEs
tOptFlags
criticalityDiagnosticsPresent = 0
mME_UE_S1AP_ID = 83939703
eNB_UE_S1AP_ID = 6926
4.6.rrcConnectionRelease
message t = 1 u c1 t = 6 u
rrcConnectionRelease(RRC连接释放)
rrc_TransactionIdentifier = 3(RRC传输ID=3)
criticalExtensions
t = 1 u c1 t = 1 u
rrcConnectionRelease_r8(RRC连接释放-R8)
tOptFlags
redirectedCarrierInfoPresent = 0
idleModeMobilityControlInfoPresent = 0
nonCriticalExtensionPresent = 0
releaseCause = 1 : ReleaseCause_Root_other(释放原因=其他)
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